You could open some 3rd-party web-sites in-game.Tutorial to open web-sites in-game.

abdallh:

2.How add in-app purchase in upbge

As @Daedalus_MDW said, there really isn’t any implementation for this yet.
However, you could make a link to a web-site, where you would have users pay online to get a password, which they could use in-game as a bonus-cheat.